The present invention is in the field of ion-doped crystals as used for optic transmission.
There currently exists a need in the optic transmission industry for an efficient amplifier in the 1.53 to 1.65 micron spectral region. The need is based on an expectation that future optic systems will utilize the full fiber transmission region from approximately 1.3 to 1.6 microns.
Current glass fiber photonic amplifiers rely on the implantation of active ions, which, possessing the proper energy level excited state structures provide optical gain, in desired wavelength regions. Current glass hosts, by their intrinsic disordered nature, exhibit low-gain cross-sections with penalties paid in optical gain and narrow optical gain bandwidth, typically from 1.53 to 1.58 microns. These low gain cross-sections predicate long lengths of glass fiber to provide amplification levels.
Quasi-three level lasers (or amplifiers) based on the ground state transitions of numerous rare-earth ions are limited by several factors. Specifically, the excited state lifetimes are too short for useful applications and the typical host matrices allow parasitic upconversion mechanisms. Once ions within a crystal are excited, they will remain excited only for a short number of milliseconds, defined as the excited state lifetime. While excited, the ion-doped crystal contains the potential energy to amplify an optic signal of similar spectral energy or resonantly pump laser light. Once the excited state lifetime expires, the ions will need to be re-excited to regenerate the potential energy. Therefore, the longer the excited state lifetimes, the less often the ions need to be re-excited and the more efficient the system.
Excited state lifetimes are largely determined by host matrix, namely by the phonon energies of the lattice. For typical oxide glasses and crystals, the lifetime is relatively short (i.e., less than 7 milliseconds). Fluoride hosts, such as yttrium lithium fluoride (xe2x80x9cYLFxe2x80x9d), have lower phonon energies and longer lifetimes (5-10 milliseconds). Increasing the first-excited-state lifetime could make efficient pulsed laser systems, such as eyesafe imaging and range finding lasers.
Parasitic upconversion processes are, to a large degree, dependent upon the host matrix. Parasitic upconversion processes deplete the excited-state population without the release of a useful photon. It is typically a concentration-dependent process since it is linked to the distances between, and relative orientation of, neighboring excited ions. Keeping the dopant levels low increases the average distance between excited ions and minimizes upconversion to some extent. However, low dopant levels are not always practical from a device perspective, and an alternate means of minimizing upconversion would be extremely beneficial. Minimization of upconversion could make optical amplifiers more efficient and smaller.
Erbium doped fiber amplifiers (EDFAs) are currently used to periodically amplify optical communications information as it passes across long fiber optic networks. These amplifiers are long (i.e., several meters) small diameter fibers doped with low concentrations of erbium (i.e., 0.2% or less). The low concentrations make erbium-erbium interactions unlikely since no two ions are likely to be near each other, whereas the long length is required to achieve significant optical gain. If the erbium concentration were increased by a factor of x, the required length would, to first order, be correspondingly reduced by a factor of x. For example a 5 meter Erbium doped fiber with a 0.2% concentration will provide gain identical to a 0.5 meter Erbium doped fiber with a 2.0% concentration. In practice, however, the latter fiber cannot be used since the dopant ions become too closely spaced in the fiber host, heavily increasing the parasitic upconversion process.
The present invention is based on the realization that calcium gallium sulfide can be used as a high gain erbium host to minimalize upconversion and increase the excited state lifetime over existing hosts.
The present invention uses a novel crystal host to minimize parasitic upconversion and lifetime quenching processes by increasing the distance between active dopant ions in the host matrix. As a result, ions such as erbium that could previously only be useful at low concentrations may now be incorporated at much higher concentrations without adverse effects. In addition, this host has long excited-state lifetimes, making it more effective if used as an energy storage device for pulsed laser applications or other applications.